The present invention relates to a three-phase permanent magnet brushless motor, and more particularly, to the structure of a reduced cogging torque powder core permanent magnet motor which is less affected by variations associated with manufacturing.
Permanent magnet brushless motors are used to drive machines for converting electric energy to mechanical power in industrial, home-electric, and automobile applications. Since importance is placed on a reduction in size of motors in addition to an improved efficiency, the reduction in size has been accomplished by an increase in density of coils in a stator core, resulting from the employment of high energy product magnets containing rare earth materials, a core division manufacturing method, and the like.
However, since the permanent magnet brushless motor has a high energy product of the magnet, a pulsatile torque called a “cogging torque” is generated due to a relationship between the number of poles of the magnet and the number of slots of the stator core. For this reason, induction motors, which do not use magnets, and coreless motors are substituted for the permanent magnet brushless motor in some cases. These motors, though generating smaller cogging torques, are inferior in energy conversion efficiency to motors based on permanent magnets. Therefore, a need exists for reducing the cogging torque of the permanent magnet motor.
A known method of reducing a cogging torque of a permanent magnet motor involves using a combination of a magnet with a stator in such a relationship to increase a least common multiple of the number of poles of the magnet and the number of slots of the stator, as shown in Japanese Patents Nos. 2954552 and 2743918. This method, when used, can reduce the period and amplitude of the cogging torque.
However, in a motor which relies on the use of a high energy product magnet to reduce the size and improve the efficiency, even if a motor is optimally designed using the combination of the number of poles with the number of slots as shown Japanese Patents Nos. 2954552 and 2743918, a lower dimensional accuracy caused by variations associated with the manufacturing can result in an increase in cogging torque.
Generally, for manufacturing a motor, a stator core is made by stamping silicon steel plates and laminating the stamped plates. Further, for increasing the density of a stator coil, it is necessary to finely divide a stator core and reassemble the divided core fragments into the stator core. In this process, a stamping accuracy, a lamination accuracy, a division/reassembly accuracy, variations in properties of the silicon steel plates, a stress applied to the core, and the like cause variations, and the stator core varies in the inner diameter dimension, magnetic properties and the like due to the resulting combination of variations associated with the manufacturing. Such variations also vary depending on particular manufacturing conditions, so that there is no guarantee that products of the same performance can be provided every time.
In applications in which the cogging torque is preferably reduced to an extreme, and in applications which require motors in homogeneous specifications, variations in cogging torque due to such variations associated with the manufacturing are not welcome in many cases. Thus, a need exists for a permanent magnet brushless motor which is capable of restraining dimensional variations and variations in magnetic properties associated with the manufacturing to reduce the cogging torque.